Electromagnetic disturbance neutralization radiation detector

ABSTRACT

A radiation detector of the Neher-White type which automatically neutralizes induced negative charges on the electrometer tube control grid which shut off the electrometer tube. The detector includes means for establishing a voltage of one polarity in response to plate current and voltage of opposite polarity in response to an absence of plate current and means for connecting the control grid to a reference potential for draining the negative charge in response to the voltage of opposite polarity.

Unite States Patent Gripentog [451 Apr. 3, 1973 [541 ELECTROMAGNETICDISTURBANCE 3,232,233 gig; E1g \BVeller ..i "gig/23.2:

rown et a gggggggg RADIATION 2,648,015 8/1953 Greenfield et al...250/83.6 R

[75] inventor: William G. Gripentog, Las Vegas, Pri ary Examiner-JamesW. Lawrence N Assistant Examiner-Harold A. Dixon Attorney-Roland A.Anderson [73] Assignee: The United States of America as represented bythe United States [57] ABSTRACT 7 Atomic Energy Commission A radiationdetector of the Neher-White type which [22] Flled: Sept- 1971automatically neutralizes induced negative charges on [21] Appl. No.:177,023 the electrometer tube control grid which shut off theelectrometer tube. The detector includes means for establishing avoltage of one polarity in response to R,328/162 plate current andvoltage of opposite polarity in [51] Int. Cl "Gilli 1/18 response to anabsence of plate current and means for [58] Field of Search ..250/83.6,83.6 R; 324/33; connecting the control grid to a reference potential328/162 for draining the negative charge in response to the voltage ofopposite polarity. 56 R i C v 1 e erences 9 Claims, 1 Drawing FigureUNITED STATES PATENTS 2,874,305 2/1959 Wilson etal ..250/83.6 R

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s I 'A' o COM- PARA r 38 REFERENCE ELECTROMAGNETIC DISTURBANCENEUTRALIZATION RADIATION DETECTOR BACKGROUND OF THE INVENTION Theinvention described herein was made in the course of under ContractAT(26-l )4l0 with the U. S. Atomic Energy Commission.

This invention relates to radiation detectors, and more particularly toradiation detectors of the Neher- White type, i.e., detectors whichemploy an ion chamber-vacuum-tube electrometer combination wherein thecontrol grid of the electrometer tube is connected only. to one of theelectrodes of the ion chamber.

Arrays of radiation detectors are frequently used to monitor the releaseof radioactivity from atomic energy activities such as the operation ofnuclear reactors or the utilization or testing of nuclear explosives.These arrays generally consist of a number of individual radiationdetectors placed around the activity being monitored with the outputsignals of the individual monitors conducted to a central control pointthrough suitable cables. In one respect the Neher-White type of detectoris ideally suited for such applications since its output is anelectrical analog of the radiation flux which is essentiallylogarithmically related thereto. Accordingly, the very wide range ofradiation intensities that could possible be seen by the monitoringdetectors can be adequately detected by a single device at eachmonitoring station without the necessity of complicated arrangements forproviding for scaling of the detector outputs.

However, the floating electrometer'control grid configuration of theNeher-White detector renders it extremely susceptible to malfunctionwhen the detector is subjected to electromagnetic disturbances ofwavelength long enough to couple inductively via the output lines intothe electrometer circuit. When such electromagnetic disturbances occur,a portion of the disturbance signal is rectified thereby depositing anextraneous negative charge on the grid of the tube. A negative charge onthe electrometer grid turns off the tube thereby preventing current fromflowing in the plate circuit. Since there is no conductive link to thefloating grid, in a very low radiation flux such a residual charge maytake hours to bleed off through the high resistance of the glassenvelope of the tube. In a higher radiation flux, the charge will bleedoff by neutralizing some of the positive ions created in the ion chamberby the radiation flux thereby causing an inaccurate output reading.

Heretofore the solution of this problem has been to use shielded cablesin the output lines in order to prevent the inductive coupling of theelectromagnetic disturbances. Since in typical arrays of radiationmonitors individual monitors may be placed at considerable distancesfrom the central control point, this approach results in very highshielded cable costs which may render the use of Neher-White detectorseconomically prohibitive for particular applications.

In those arrays where Neher-White detectors have been used, withshielded cables providing the protection against electromagneticdisturbances, it has been quite common for a relay to have been includedin each of the individual detectors. The purpose of those relays hasheretofore been limited to providing the means for making remote checkson (and readjustment of) the electrical characteristics of the detectorcircuit against previously established calibration values, the functionof the relay being to connect the electrometer control grid directlyinto the plate circuit upon activation by a manually operated switch.

I have discovered, however, that these relays can be utilized toneutralize the effects of electromagnetic disturbances since aconnection of the electrometer grid to the plate circuit provides adrain path for a negative charge on the grid, and that, consequently,shielded cables need no be used in these detector arrays. Accordingly,my invention concerns means and method of automatically activating arelay in a Neher- White detector for providing a drain path for anegative charge deposited on the electrometer grid, therebyautomatically neutralizing the effects of electromagnetic disturbanceson the detector.

SUMMARY OF THE INVENTION Accordingly, it is an object of the inventionto provvide a radiation detector of the Neher-White type wherein anextraneous negative charge on the grid of the electrometer tube isautomatically neutralized. Briefly summarized, the above and additionalobjects are accomplished by automatically connecting the control grid toan appropriate reference potential for draining the extraneous charge inresponse to a cessation of current flow in the plate circuit of theelectrometer tube. This is accomplished by establishing a voltage of onepolarity in response to current flow in the plate circuit of theelectrometer tube and a voltage of opposite polarity in response to anabsence of current flow therein, and connecting the control grid tomeans for draining the negative charge thereon in response to thevoltage of opposite polarity. A particular combination of circuitelements for performing these functions includes an operationalamplifier in the plate circuit for establishing the voltages of one andopposite polarities and a circuit connecting the output of theoperational amplifier with an integral relay in the ionchamber-electrometer tube combination with a circuit element interposedtherein which passes energization current to the relay in response tothe voltage of opposite polarity only.

Additional objects, advantages, and details of the invention will bemore fully understood after consideration of a preferred embodiment ofthe invention as described below in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of drawing is acircuit diagram illustrating a preferred embodiment of an apparatus forpracticing the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawing, ionchamber C is represented by outer positive electrode '10, which isusually the outer shell of the ion chamber, and central negativeelectrode, or collector, l2. Collector 12 is connected directly tocontrol grid 14 of vacuum tube electrometer V, which is connected as atriode, as is generally typical in Neher-White detector arrangements.The collection potential for ion chamber C is provided by battery Blconnected to outer electrode and common, or reference, lead 16. Thecombination of ion chamber C with integral electrometer tube V and relayK, which will be referred to later in connection with its function, areavailable as standard items from the Jordon Nuclear Company of Pasadena,Calif. and the Ralph White Company of Altadena, Calif. These ionchamber-electrometer tube-relay combinations have been extensively usedin nuclear reactor monitoring arrays wherein shielded wires wereutilized to negate the effects of electromagnetic disturbances. Theutility of relay K in these applications was limited to providing ameans for shorting out the electrometer I control grid so that theelectrical components of the detector circuit minus the ion chamberinput could be subjected to calibration checks and adjustments. Anexample of the use of relay K in this manner will be describedhereinafter.

The plate circuit for electrometer tube V includes lead 17, operationalamplifier 18, the function of which will be described hereinafter, lead20, and the cathode and plate of tube V. Cathode filament voltage andthe potential between the cathode and plate of tube V are provided bythe filament heater circuit which includes lead 22, adjustable resistorR1, and battery B2 in addition to lead 20. The ionization of the gas inchamber C by incident radiation and the consequent migration of positiveions to collector 12 makes control grid 14 of electrometer tube V morepositive, i.e. its negative bias is reduced, thereby allowing current toflow in the plate circuit. The magnitude of that current flow is anessentially logarithmetic analog of the intensity of the ionization inchamber C.

With relay K in its deenergized posture, as shown in the drawing,collector electrode 12, and therefore control grid 14, are floating,i.e., are electrically insulated from all other circuits of the system.As indicated earlier, this floating collector-control grid of the Neher-White ion chamber-electrometer tube combination is particularlysusceptible to malfunction due to the deposition and retention ofnegative charges on the collector-control grid. When electromagneticdisturbances of wave length long enough to couple inductively via theoutput lines into the electrometer circuit are experienced, a portion ofthe disturbance signal is rectified resulting in the deposition of anegative charge on the control grid of the tube. A negative charge onthe electrometer control grid turns the tube off allowing no current toflow in the plate circuit. To negate this effect, the turned offcondition of electrometer V is sensed and relay K automaticallyactivated to provide a conductive path for bleeding off that negativecharge and then deactivated to return the electrometer to normaloperation. The manner in which this is accomplished will now beexplained.

The output of the plate circuit is fed into summing junction, ornegative terminal, 24 of operational amplifier 18 which, in accordancewith well known characteristics of operational amplifiers, produces acurrent output equal in magnitude to, but opposite in polarity from, theinput into the summing junction. The output of operational amplifier 18is fed back to summing junction 24 through the feedback circuitincluding lead 26 and variable resistor R2. The magnitude of the outputof the operational amplifier than increases or decreases in response toincreases and decreases in the electrometer plate current to produce azero input at summing junction 24. Variable resistor R2 is set at avalue which provides a selected voltage drop across the operationalamplifier when the ion chamber is subjected to a selected level ofradiation thereby providing calibration of the detector. The output ofthe detector may be taken off as a voltage across output lead 30 andreference lead 16 or as a current across leads 30 and 32 (with, ofcourse, the removal of R2).

A third input to summing junction 24 is from the zero adjust loopincluding lead 28 and variable resistor R3. Resistor R3 is set at avalue which provides that positive current input (from battery B3) tosumming junction 24 which will just offset the negative input fromelectrometer V when ion chamber C is subjected to some selected level ofradiation, such as average background. Accordingly, the input into andthe output from operational amplifier 18 at that selected level ofradiation will be zero. When electrometer V is shut off by deposition ofa negative charge on collector 12, the only input into operationalamplifier 18 will be the positive current of this zero adjust loop.Accordingly, the output of the operational amplifier will be a negativecurrent of magnitude equal to that of the zero adjust loop when tube Vis shut off.

It can be ,seen then, that the use of operational amplifier 18 in theplate circuit provides a means for sensing the turned off condition ofthe electrometer since the output of the operational amplifier switchespolarity in response to that turned off condition. While a preferredcombination of circuit elements for utilizing this reverse in polarityof the output for energizing relay K is shown in the drawing and will beexplained hereafter, it will be appreciated that the basic requirementis that the connection of the operational amplifier output to the relaybe made through a circuit element which permits an energization signalto reach relay K only in response to a negative output. A diode is, ofcourse, the simplest circuit element which meets this requirement.

It is worth noting at this point that the operational amplifier providesadvantages in addition to those primarily related to providing the meansfor sensing the turned off condition of the tube. The operationalamplifier maintains the plate of the electrometer at a virtual groundthereby insuring a constant potential between the plate and cathode ofthe electrometer tube regardless of the output line resistance. Thisimproves the linearity of the detector response and makes the ability todrive the output over highimpedance output lines the function of thevoltage swing of the amplifier which in turn is a function of theamplifier supply voltage. Supply power is provided by series connectedbatteries B3 and B4 which may or may not be equal depending upon thedesired symmetry of the operational amplifier output. The splitting. ofthe supply power determines the potential of reference lead 16.Regulation of the voltage outputs of batteries B3 and-B4 may beaccomplished by any convenient means known in the art.

Referring now to the preferred arrangement for some convenient point inthe circuit, not shown, (such as the junction of lead with the regulatedoutput from battery B4). Resistor R4 establishes the value of thecomparator reference voltage. When the magnitude of the voltage outputfrom operational amplifier l8, i.e., the input into the comparator,exceeds that reference, comparator 34 produces a constant magnitudeoutput which is of the same polarity as its input. The output ofcomparator 34 is transmitted to transistor amplifier Q and when it isnegative, i.e., when the electrometer V is shut off as previouslyexplained, it is amplified and passed by transistor Q over lead 40thereby energizing relay K. This connects collector 12- control grid 14to lead 20 of the plate and cathode circuits through the loop includingresistor R5 thereby providing a drain path for the negative chargethereon and placing the grid at cathode voltage which causes current toflow in the electrometer plate circuit. This current flow in the platecircuit adds a negative input into summing junction 24 of operationalamplifier 18 thereby producing a positive output which changes thepolarity of the input to comparator 34 over lead 36. Accordingly, sincethe output from comparator 34 is now positive, the output of transistorQ falls to zero thereby permitting relay K to return to its deenergizedposture as illustrated in the drawing which, in turn, returns thedetector to its normal operating mode.

Utilization of the comparator as the element which passes theenergization signal to relay K provides several advantages over the useof a diode. The comparator permits the switching point to be preciselydefined whereas the switching point using a diode would be dependentupon the junction potential of that particular diode, the junctionpotential varying from diode to diode and being subject to changes intemperature. This more precise definition of the switching point is ofimportance in minimizing the probability of relay K being energized inresponse to natural variations in incident radiation below averagebackground (as compensated by the setting of R3). In addition, the useof the comparator enables the same switching point to be established foreach of the detectors of an' array of detectors.

Rapid readjustment of the electrical characteristics of the circuit tocalibration values is facilitated by resistors R6 and R7 which may beselectively inserted in the circuit by manipulation of double pole,double throw switch S. As indicated earlier, resistor R2 is adjusted sothat a convenient output of the operational amplifier is obtained whenthe ion chamber is subjected to a selected calibration intensity ofradiation and resistor R3 is adjusted so that the output of theoperational amplifier is zero when ion chamber C is subjected tobackground radiation. When these adjustments are completed, switch S ismanipulated so that pole 42 places R6 in parallel with R3 and pole 44makes contact with one of the terminals 46 or 48 to turn on transistor 0thereby energizing relay K. As long as switch S is maintained in thatposture, control'grid 14 of electrometer V is at cathode voltage therebyallowing a large current flow in the plate circuit essentially as adiode. R6 is then adjusted so that a convenient output voltage value isobtained across output lines and 16, and that output is recorded. SwitchS is then manipulated to place R7 in parallel with R2, the

other terminal of the pair 46, 48 also energizing relay K. As before, R7is adjusted to provide a convenient output which is recorded. At somelater time, the electrical characteristics can be checked andreadjusted, if necessary, by returning switch S to the positions justdescribed and adjusting resistors R2 and R3 if any variance from therecorded outputs is observed.

While the fundamental novel features of the invention have now beenshown and described and pointed out as applied to an embodimentparticularly described, and further variations have been suggested andindicated, it will be readily appreciated by those skilled in the artthat various omissions and substitutions and changes may be made withinthe principle and scope of the invention as expressed in the appendedclaims.

What is claimed is: 1. In a radiation detector of the type utilizing anion chamber and vacuum-tube electrometer combination wherein the controlgrid of the electrometer tube is normally connected only to one of theelectrodes of the ion chamber, a combination of electrical circuitelements which automatically remove an extraneous negative charge fromsaid control grid comprising:

a plate circuit connected to the electrometer, means for establishing avoltage of one polarity with respect to a reference potential inresponse to current flow in said plate circuit and a voltage of opposite polarity with respect to said reference in response to acessation of said current flow, and

means for making a conductive connection between said control grid and adrain path for said extraneous charge in response to said oppositepolarity voltage.

2. The combination of claim 1 wherein said means for establishing saidvoltages of one and opposite polarities include an operational amplifierthe summing junction of which is connected to the plate of saidelectrometer and the output of which is connected to said means formaking said conductive connection.

3. The combination of claim 2 wherein the input to said summing junctionincludes a positive polarity current equal in magnitude to the negativecurrent input from said plate when said ion chamber is subjected to aselected level of radiation.

4. The combination of claim 3 wherein said selected level of radiationis background radiation.

5. The combination of claim 1 wherein said means for making a conductiveconnection between said control grid and said drain path include a relayand circuit means for passing an activation signal to said relay inresponse to said opposite polarity voltage.

6. The combination of claim 3-wherein said means for making saidconductive connection include a relay which upon activation anddeactivation connects and disconnects said control grid to and from apoint in the detector circuit at an appropriate potential and circuitmeans which include a combination of a comparator and transistor whichpasses an activation signal to the relay only in response to negativeoutput from the operational amplifier greater than a preselectedmagnitude.

7. A method of providing automatic neutralization of the effects ofelectromagnetic radiation in shutting off the electrometer of aradiation detector of the type tube, and producing a signal foractivating the relay in response to said voltage of opposite polarity.

8. The method of claim 7 wherein said voltage of one polarity and saidvoltage of opposite polarity are the output of the detector andincluding the step of comparing said output against a preselectedreference value prior to producing said activating signal.

9. The method of claim 8 including the step of inactivating said relayin response to a turned on condition of said electrometer.

1. In a radiation detector of the type utilizing an ion chamber andvacuum-tube electrometer combination wherein the control grid of theelectrometer tube is normally connected only to one of the electrodes ofthe ion chamber, a combination of electrical circuit elements whichautomatically remove an extraneous negative charge from said controlgrid comprising: a plate circuit connected to the electrometer, meansfor establishing a voltage of one polarity with respect to a referencepotential in response to current flow in said plate circuit and avoltage of opposite polarity with respect to said reference in responseto a cessation of said current flow, and means for making a conductiveconnection between said control grid and a drain path for saidextraneous charge in response to said opposite polarity voltage.
 2. Thecombination of claim 1 wherein said means for establishing said voltagesof one and opposite polarities include an operational amplifier thesumming junction of which is connected to the plate of said electrometerand the output of which is connected to said means for making saidconductive connection.
 3. The combination of claim 2 wherein the inputto said sumMing junction includes a positive polarity current equal inmagnitude to the negative current input from said plate when said ionchamber is subjected to a selected level of radiation.
 4. Thecombination of claim 3 wherein said selected level of radiation isbackground radiation.
 5. The combination of claim 1 wherein said meansfor making a conductive connection between said control grid and saiddrain path include a relay and circuit means for passing an activationsignal to said relay in response to said opposite polarity voltage. 6.The combination of claim 3 wherein said means for making said conductiveconnection include a relay which upon activation and deactivationconnects and disconnects said control grid to and from a point in thedetector circuit at an appropriate potential and circuit means whichinclude a combination of a comparator and transistor which passes anactivation signal to the relay only in response to negative output fromthe operational amplifier greater than a preselected magnitude.
 7. Amethod of providing automatic neutralization of the effects ofelectromagnetic radiation in shutting off the electrometer of aradiation detector of the type having an ion chamber and vacuum tubeelectrometer combination wherein the control grid of the electrometer isnormally connected only to one of the electrodes of the ion chamber andincluding a relay which upon activation provides a conductive pathbetween the control grid and detector circuitry, the steps comprisingestablishing a voltage of one polarity with respect to a referencepotential in response to normal operation of the electrometer tube,establishing a voltage of opposite polarity with respect to saidreference in response to a turned off condition of the electrometertube, and producing a signal for activating the relay in response tosaid voltage of opposite polarity.
 8. The method of claim 7 wherein saidvoltage of one polarity and said voltage of opposite polarity are theoutput of the detector and including the step of comparing said outputagainst a preselected reference value prior to producing said activatingsignal.
 9. The method of claim 8 including the step of inactivating saidrelay in response to a turned on condition of said electrometer.